1. Field of the Invention
This invention relates to the purification of esters and especially the purification of diaryl esters of dicarboxylic aromatic acids, such as diphenyl phthalates and dicresyl phthalates.
2. Review of the Prior Art
The production of various phenolic esters of aromatic carboxylic acids, such as diphenyl terephthalate and diphenyl isophthalate, has become of significant commercial interest in recent years due to their use in a great many types of processes. For example, diphenyl terephthalate and diphenyl isophthalate dissolved in a solvent may be reacted with a primary diamine to produce polyamides. However, despite the importance of these phenolic esters, no processes have been developed for their production that are satisfactory as to both price and performance. For example, expensive aroyl reactants can be used to prepare the esters, or multiple distillations may be employed for product purification, but the products are then overpriced for market usage.
Crude diphenyl phthalates made by direct esterification or by exchange reactions are pink to brown colored, having an APHA color of up to 500 and acid no. 4, for example, acid number being defined as mg KOH/g. Crude diphenyl phthalates are difficult to purify to the extent needed for many applications such as the production of polycondensation products and particularly high quality resins. These uses require diphenyl phthalates which are colorless or at least of low color (e.g., APHA 10 and at least less than APHA 40) and have a low acid number (e.g., acid no. 0.01 and at least less than 0.2 mg KOH/g.)
This coloration cannot be completely removed, thereby obtaining specification quality diphenyl phthalates, by vacuum distillation, by recrystallizing the ester from solvents such as aromatic hydrocarbons, or by using adsorbents, such as activated carbons, charcoal, or activated fullers' earth.
Methods involving the addition of a solvent for purifying the ester, followed by subsequent phase separation, have been troubled with formation of emulsions and gelatinous precipitates. Other, methods involving heat transfer, such as vacuum distillation, create additional difficulties because of the very high melting point of the diphenyl terephthalate of 199.degree.-200.degree. C.
One purification technique for the reaction product of aromatic carboxylic acids with aliphatic alcohols, as described in U.S. Pat. No. 4,304,925, comprises the addition of water at 5-50% by weight, heating at reflux until the organo titanium compound used as catalyst is hydrolyzed, adding a basic substance, such as hydroxides, carbonates, or bicarbonates of alkali metals, separating the esterification reaction mixture from the aqueous layer, further purifying, if desired, by washing with water and subsequently separating the aqueous layer, and distilling under reduced pressure or at elevated temperatures or by treating with activated clay, activated carbon, diatomaceous earth, or the like. The aromatic carboxylic acids include phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, hemimellitic acid, and trimesic acid.
A difficulty with such a purification method is that the ester product must be cooled below 100.degree. C., thereby often causing phase separation difficulties between the ester phase and the aqueous phase with consequent loss of product on phase separation. Particularly when titanium compounds are used as esterification catalysts, the use of aqueous alkali hydrolyzes the metal compound to produce a gelatinous precipitate which tends to aggravate the loss of product when organic or aqueous phases are separated. Furthermore, if alcohol removal is attempted by blowing steam through the reaction product at a temperature in excess of 100.degree. C. without first removing the esterification catalyst, a titanium catalyst can be hydrolyzed to a difficultly separable gel, as discussed in U.S. Pat. No. 3,818,071.
A method for purifying a solution of dimethyl terephthalate in a water-insoluble organic solvent, such as chloroform, comprises passing the solution through a column in concurrent or countercurrent flow to a dilute aqueous solution of sodium hydroxide, as taught in U.S. Pat. No. 2,753,369.
U.S. Pat. No. 2,656,377 teaches the separation of isophthalate and terephthalate di-esters from a mixture thereof by adding any two different solvents capable of dissolving different ratios of the isophthalate and the terephthalate. Xylene is a suitable solvent of lower solubility ratio.
U.S. Pat. No. 3,277,153 relates to a process for the preparation of diphenyl carboxylic acid methyl esters from distillation residues obtained in the production of tere- and/or isophthalic acid dimethyl esters. After re-esterification, the residues are dissolved in hot xylene or methanol, and the diphenyl carboxylic acid methyl esters are obtained by repeated crystallization from the solvent.
U.S. Pat. No. 3,600,430 relates to a process for purification of a diester of a benzenedicarboxylic acid, e.g., bis(2-hydroxyethyl) terephthalate, by adding thereto mixed xylene solvent (Example II). The preferred amount of solvent is such that the ratio of diester dissolved in the solvent, i.e., the lighter phase, to the diester in the heavy or molten diester phase is less than about 48:1 and preferably less than about 24:1. The phases are separated, and purified diesters are recovered from the lighter phase.
A specific purification method is disclosed in U.S. Pat. No. 3,705,186 for producing pure, colorless diphenyl terephthalate. It begins with transesterification of dialkyl terephthalates with at least equivalent amounts of phenyl acetate in the presence of butyl titanate as a catalyst by (a) heating the dialkyl terephthalate with phenyl acetate in an inert atmosphere in the presence of 1-5 wt. % of activated carbon to temperatures above 150.degree. C., (b) then adding the titanic acid ester, (c) immediately removing the alkyl acetate, (d) stirring the hot carbon-containing crude ester into a relatively high boiling aromatic hydrocarbon, such as xylene, under an inert gas, and (e) crystallizing the product after removing activated carbon by filtering.
U.S. Pat. No. 4,464,477 relates to a process for recovery and reuse of heavy metal oxidation catalyst from distillation residues in the production of dimethyl terephthalate by oxidation of p-xylene and methyl p-toluate. The distillation residue feed is mixed with aqueous extractant. The resulting emulsion is settled, and the organic phase therefrom is mixed with the waste water extractant from the oxidation process and settled again to obtain the extractant as the aqueous phase. Xylenes can be added to the organic phases in order to lower viscosity.
All of these purification methods involve losses of product, difficulties with filtering or phase separation, or the use of expensive additives such as activated carbon. Accordingly, there is a need for a simpler, easier, and less expensive purification process for diaryl esters such as diphenyl phthalates.